As broadly known, normally a crystal oscillator (a circuit configured by a crystal resonator and an oscillation circuit) is used for an electronic apparatus such as a wireless communication apparatus and a computer, however, an oscillation frequency of the crystal resonator changes depending on a temperature. Therefore, a TCXO (Temperature Compensated Crystal Oscillator) incorporating the crystal resonator and a temperature compensation oscillation circuit into one package is used for the apparatus that particularly requires an accurate frequency.
The general TCXO is configured to compensate the temperature on the basis of a temperature (measured by a thermo-sensitive element provided in the vicinity of the crystal resonator) ambient to the crystal resonator, however, a crystal oscillator compensating the temperature on the basis of a temperature of the crystal resonator itself is also developed.
To be specific, as illustrated in FIG. 1, the crystal resonator has a plurality of vibration modes (a thickness-shear vibration mode and a contour type vibration mode in FIG. 1) with frequency temperature characteristics being different from each other. Accordingly, temperature information (that can be converted into the temperature of the crystal resonator) on the crystal resonator can be acquired by obtaining a difference between the oscillation frequencies in the two vibration modes of the crystal resonator.
Therefore, as depicted in FIG. 2, a crystal oscillator (refer to, e.g., Patent document 1) is developed, which compensates the temperature by connecting two oscillation circuits to the crystal resonator, generating beat signal of outputs of the two oscillation circuits with a mixer, extracting a low frequency component containing the difference between the oscillation frequencies from the generated beat signal by use of a lowpass filter and obtaining the temperature information on the crystal resonator and thus compensating the temperature on the basis of the temperature information.